This invention relates to a welding power supply operated from a three phase power input for use in inert gas arc welding, and more particularly to such a supply wherein independent numbers of consecutive half cycles of each polarity of the three phase power input are passed to the welding arc to generate an asymmetrical arc current.
The use of square wave bipolar arc currents is well known in the prior art. Such currents are particularly useful for inert gas arc welding using nonconsummable electrodes, such as tungsten, commonly referred to as tungsten inert gas (TIG) welding. The use of a square wave bipolar welding current provides a cleaning action of the workpiece with the reverse polarity current (electrode positive relative to the workpiece) while providing maximum welding efficiency via straight polarity current (electrode negative relative to the work piece).
For each polarity reversal, the arc current goes through zero and, accordingly, the arc must be reignited to provide currrent flow for the reversed polarity of arc current. Current reversals can create reignition problems when the reversal is attempted at low voltage levels. Arc reignition is a particular problem for current reversals from straight polarity to reverse polarity since the relatively cool base metal is not as good an emitter as the hot tungsten electrode. Even for current reversals from reverse polarity to straight polarity, it is advantageous to perform the current reversal at high voltage levels to insure arc reignition.
The problem of arc reignition in square wave welding has been handled for example by silicon controlled rectifier (SCR) circuits wherein SCR conduction is not initiated until the welding supply voltage level reaches a defined magnitude or defined phase angle relative to the zero crossing point. See, for example, U.S. Pat. No. 3,845,380 which describes a current stabilizer wherein current transitions are performed by SCR's activated by saturable reactors so that the current transitions occur at non-zero levels of the welding supply voltage.
Other known welding supply circuits provide arc current reversals by switching between two independent direct current (DC) power supplies by commutating SCR's or by connecting a single DC power supply to an arc via a commutated SCR bridge circuit. See, for example, U.S. Pat. Nos. 3,999,034 and 4,180,720 which are assigned to the assignee of the present application. These welding supplies further provide the benefit of generating an asymmetrical current waveform, i.e., adjusting the duty cycle so that straight polarity current is applied to the arc for a different portion (normally greater portion) of each cycle than reverse polarity current. By adjusting the duty cycle, a selected amount of "cleaning" arc current or reverse polarity arc current can be provided depending upon the composition and condition of the workpiece. This permits a maximum portion of straight polarity current to be provided to the arc to more efficiently perform the weld. While these welding power supplies provide versatility and superior welding performance, the commutation circuits for the SCR's are expensive.
An asymmetrical single phase alternating current welding supply is provided in accordance with U.S. Pat. No. 4,038,515 by independently selecting the non-zero voltage points at which the straight and reverse polarity currents are initiated, i.e., the time phases at which SCR bridge paths are made to conduct. The independent selection of the non-zero voltage points for initiation of the straight and reverse polarity currents generates an asymmetrical arc current waveform by imbalancing the conduction angles for the two polarities of arc current. However, imbalancing the conduction angles of the straight and reverse polarity currents to the arc leads to partial saturation of the power transformer. Such transformer saturation causes high primary current flow which includes a reactive current component, generates harmonics on the power line, creates a low power factor, and produces heating in the power transformer primary and the distribution transformer, all of which result in inefficient operation and higher billing charges from the power distributing company.